The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
An aircraft propulsion unit comprises conventionally a turbojet engine housed inside a nacelle.
The nacelle generally has an annular structure comprising an air inlet upstream of the turbojet engine, a median section intended to surround a fan of said turbojet engine and its casing, and a downstream section intended to surround the combustion chamber of the turbojet engine and enclosing thrust reversal means where necessary. It can be ended with an ejection nozzle, the outlet of which is situated downstream of the turbojet engine.
The air intake structure is used to optimize the capture of air required for the supply of the fan of the turbojet engine and to channel it toward this fan.
An air intake structure comprises, particularly upstream, a leading edge structure commonly called air intake “lip”.
The air intake lip ensures capture of the air and is fastened to the rest of the air intake structure that ensures the channeling of the air captured toward the turbojet engine.
To this end, the rest of the air intake structure has a substantially annular structure comprising an external panel ensuring the external aerodynamic continuity of the nacelle and an internal panel ensuring the internal aerodynamic continuity of the nacelle, in particular with the fan casing at the median section. The air intake lip ensures the upstream junction between these two walls and can be particularly integrated with the external panel. It also ensures de-icing or anti-icing of the nacelle, by applying heat to melt and evaporate ice that may deposit therein.
All of the air intake structure is fastened upstream of the median section of the nacelle (external panel) and of the fan casing (internal panel). The absorption of forces transiting through the air intake is particularly ensured by a fastening flange to the fan casing.
The internal surface of the air intake structure is exposed to a significant air flow and is situated near the fan blades. It is therefore situated in a significant noise zone.
In order to remedy this situation insofar as possible and in order to reduce the noise pollution generated by the turbojet engine, the internal panel of the air intake section is equipped with an acoustic attenuation structure.
This acoustic attenuation structure is in the form of a sandwich panel with a cellular core having a holed external skin, called acoustic skin, intended to be exposed to noise, and a solid internal skin. The cellular core thus constitutes a resonator adapted to trap sound waves.
As for the air intake lip it is not equipped with an acoustic attenuation structure for structural and thermal reasons, and it is therefore necessary to provide connections at the junction between the internal panel and the air intake lip. Due to the thickness of the acoustic attenuation structure, this connection presents many implementation difficulties.
There are several possibilities to ensure the transition between the internal panel equipped with the acoustic attenuation structure and the air intake lip.
First of all, it is possible to provide connections at the acoustic skin between the internal panel and the air intake lip.
There are also solutions with solid and acoustic operational skins but calculations of force paths and of dimensioning are more complex.
In the conventional solutions, the air intake lip is made from a non-acoustic skin (solid) and the internal panel has a solid skin (also called “backskin” or rear skin) of its acoustic panel, which is structural. The connection is made by means of a structural crank ensuring mechanical connection and the transmission of the forces between said back skin and the air intake lip despite the offset due to the thickness of the acoustic panel. Because of these geometric constraints, the crank is structurally difficult to achieve and thus impacting the mass of the unit. Moreover, it prevents the extension of the acoustic panel and the extension of its performances to the structure of the air intake lip.
There is therefore a need to improve the structural strength of such an air intake structure and to allow the use of at least part of the air intake lip zone in order to improve the acoustic performances by providing it with an acoustic attenuation structure extending the one of the internal wall.